The present invention relates generally to apparatus and methods for measuring optical distortion in transparent parts, and more particularly for measuring distortion in transparencies through which visual tasks are performed.
Optical distortion is one of several factors used to assess the quality of transparencies, such as aircraft transparencies, automobile windshields and helmet visors. The amount of optical distortion in a transparency is commonly determined using one of three measurements, viz., displacement grade, lens factor or grid line slope. Each measurement is performed on an image viewed through the transparency of a one inch grid board, is normally of a very small distance, and requires a significant amount of judgment. The subjective nature of the measurements therefore often results in different operators obtaining different distortion values for the same transparency (see, e.g., Kama, "Measures of Distortion: Are They Relevant?", Conference on Aerospace Transparent Materials and Enclosures, Vol II, S. A. Marolo, Ed, Rept #WRDC-TR-89-4044 (April 1989).
The invention solves or substantially reduces in critical importance problems with existing optical distortion measuring methods as just suggested by providing an accurate, reliable and automated distortion measuring method utilizing digital imaging. The invention utilizes a video camera and a frame grabber installed in a microcomputer to digitize images of a grid board. The frame grabber digitizes the analog signal from the video camera and relevant information from the image is condensed into an array containing information on the separation, location and shape of each grid line.
It is therefore a principal object of the invention to provide a method for measuring optical distortion in a transparency.
It is a further object of the invention to provide an optical distortion measuring method which is substantially free of operator error.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.